Pin without head HR-18152 - Tin Bronze - Late Bronze Age - Switzerland

Pin without head HR-18152 - Tin Bronze - Late Bronze Age - Switzerland

Pin without head HR-18152

Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

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Credit HE-Arc CR.

Fig. 1: Pin without head (after Rychner-Faraggi 1993, plate 74.29),

Credit HEI Arc, S.Ramseyer

Fig. 2: Dense and smooth brown-yellow patina (detail) of the pin with lacuna showing the underlying metal with location of Fig. 4,

Credit

Fig. 2:

Pin without head and smooth brown-yellow patina typical of lake context (Figs. 1-2). Dimensions: L = 7,5cm; Ø = 3.3mm; WT = 4g.

Pin

Hauterive - Champréveyres, Neuchâtel, Neuchâtel, Switzerland

Excavation 1983-1985, object from layer 3 to 5

Late Bronze Age

Hallstatt B1 (1054/1037BC _ 1000BC)

Lake

Laténium, Neuchâtel, Neuchâtel

Laténium, Neuchâtel, Neuchâtel

Hr 18152

Not conserved

Complementary information

Smooth and dense brown-yellow patina has been extensively described by Schweizer (Schweizer 1994).

Credit HEI Arc, C,Cevey.

Fig. 3: Side view of a lacuna with the stratigraphy of the brown-yellow patina.

Credit HEI Arc, S.Ramseyer.

Fig. 4: SEM image of the pin from Fig. 2 (detail), BSE -mode. The crack between the remaining metal and the corrosion layers is clearly visible and the smooth layer below is CP3,

Credit HE-Arc CR.

Fig. 5: Location of sampling area,

Credit HEI Arc, C,Cevey.

Fig. 6: Cross-section of the pin and remaining material,

The schematic representation below gives an overview of the corrosion layers encountered on the pin from a first visual macroscopic observation.

Credit HEI Arc, N,Gutknecht.

Fig. 7: Stratigraphic representation of the pin without head by macroscopic observation (under binocular),

Fig. 8: Stratigraphic representation of the pin without head under binocular using the MiCorr application. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation, Credit HE-Arc CR.
Credit HE-Arc CR.

Fig. 9: Micrograph of the cross-section of the sample taken from the pin without head showing the location of Figs. 11 to 18,

The cross-section is circular and is a complete section through the pin (Figs. 5 and 6). It is covered with a rather thin and regular (in thickness) corrosion crust (Figs. 3 and 4). One third of the corrosion crust is missing (Fig. 9). 

Tin Bronze

Annealed after cold working

MAH 87-194

Musées d'art et d'histoire, Genève, Geneva

Musées d'art et d'histoire, Genève, Geneva

1987, metallography and corrosion characterisation

Complementary information

This sample is mentioned in Schweizer, 1994.

Analyses performed:
Metallography (etched with ferric chloride reagent), Vickers hardness testing, XRF, ICP-OES, SEM/EDS, XRD, Raman spectroscopy.

The remaining metal is a tin bronze and contains copper sulphide as well as heavy metal (Pb-rich) inclusions (Table 1, Figs. 11 and 12). Close to the surface of the remaining metal, copper sulphide inclusions are elongated and form rows (Fig. 11). The etched structure of the tin bronze shows polygonal grains; some of them are twinned (Fig. 13). In the centre of the sample and on the edges, the grains are smaller. The copper sulphide inclusions are located at the grain boundaries and in the grains. The average hardness of the metal is about HV1 110.

Elements Cu Sn Pb Sb As Ag Fe Ni Co Zn
mass% 89.22 9.57 0.34 0.26 0.19 0.15 0.09 0.05 0.06 0.05

Table 1: Chemical composition of the metal. Method of analysis: ICP-OES, Laboratory of Analytical Chemistry, Empa.

Credit HE-Arc CR..

Fig. 11: Micrograph of the metal sample from Fig. 9 (detail), unetched, bright field. Rows of elongated copper sulphide inclusions can be observed,

Credit HE-Arc CR.

Fig. 12: SEM image of the metal sample from Fig. 9 (detail), SE-mode. Elongated copper sulphide inclusions (dark-grey) are visible as well as heavy metal (Pb-rich) inclusions (white),

Credit HE-Arc CR.

Fig. 13: Micrograph of the metal sample from Fig. 9 (detail), etched, bright field. The metal shows a structure of polygonal and twinned grains. Copper sulphide inclusions are visible as blue spots,

Polygonal and twinned grains

Cu

As, Ag, Sn, Sb, Pb

Complementary information

Schweizer (1994) indicates that the copper-tin alloys similar to the one of the pin have minor constituents that were certainly not added intentionally. Furthermore, he mentions that there is no systematic composition difference between bronzes with a lake patina and those with a land patina.

The corrosion crust (patina) is regular in thickness (around 50µm, Fig. 4). It presents lacuna (Fig. 2) and in some areas one third is missing (Fig. 9). At the metal - corrosion crust interface, there is a crack showing that the latter has separated from the metal core along its whole length (Figs. 4, 9, 11, 14-15). The corrosion crust can be divided into three distinct layers (CP1-3). Directly above the crack is a first dense but cracked and irregular inner layer (CP3, Figs. 4, 14-15). In bright field it appears brown (Fig. 16), in normal and polarised light dark brown (Figs. 2 and 17). It is separated from the adjacent layer by a clear line (Figs. 15 and 16). The second layer (CP2) is dense with little porosity (Figs. 14 and 15). In bright field it appears light brown (fig. 16), in polarised dark dark yellow (Fig. 17). The third and outermost layer (CP1) appears light brown under normal light (Fig. 3) and in bright field (Fig. 16), contains particles (Fig. 15) and is very porous (visible as golden reflections under polarized light, Fig. 17). These results are entirely consistent with Schweizer's observations (Schweizer 1994). The elemental chemical distribution of the SEM image selected reveals that the inner layer (CP3) is depleted in Cu, but rich in Sn,O and Si (Fig. 18 and Table 2) and its interface with the intermediate layer (CP2) could represent the limit of the original surface (Figs. 15 and 18). The second and third layer (CP2 and CP1) are Fe, Cu and S-rich (Fig. 18) and have a composition similar to chalcopyrite/CuFeS2 (Table 2). This was confirmed by XRD. The particles (inclusions) have a composition similar to covelline or covellite/CuS (Table 2). Both chalcopyrite and covelline have been identified by Raman spectroscopy (Figs. 19 and 20).

Elements

 S Fe Cu O Si Sn Total
CP1 and CP2 35 30 34 < < < 99
Particles in CP1 26 4.1 68 < < < 98
CP3 5.8 5.0 13 32 2 41 99

Table 2: Chemical composition (mass %) of the corrosion layer from Fig. 16. Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Credit HE-Arc CR.

Fig. 14: SEM image of the metal sample from Fig. 9 (detail), SE-mode. The crack between the remaining metal and the corrosion layers is clearly visible,

Credit HE-Arc CR.

Fig. 15: SEM image similar to Fig. 14, BSE-mode. A thin line is visible (white arrow) differentiating the inner Sn-rich layer from the intermediate and outer chalcopyrite layers. The latter contains bright covelline inclusions (black arrow) and pores,

Credit HE-Arc CR.

Fig. 16: Micrograph of the metal sample from Fig. 9 (detail) and corresponding to the stratigraphy of Fig. 4, unetched, bright field. The area selected for elemental chemical distribution (Fig. 18) is marked by a red rectangle,

Credit HE-Arc CR.

Fig. 17: Micrograph similar to Fig. 16, polarised light,

Credit Empa.

Fig. 18: SEM image and elemental chemical distribution of a selected area of Fig. 16 (rotated by 90°). Method of examination: SEM/EDS, Laboratory of Analytical Chemistry, Empa,

Credit SNM.

Fig. 19: Raman spectrum of the outermost layer (S23) of Fig.16 compared to a chalcopyrite standard spectrum. Settings: laser wavelength 532nm, acquisition time 50s, 4 accumulations, filter D2 (0.75-0.8mW), hole 1000, slit 100, grating 600. Method of analysis: Raman spectroscopy, Lab of Swiss National Museum, Affoltern a. Albis ZH,

Credit SNM.

Fig. 20: Raman spectrum of the inclusions of the outermost layer (S42) of Fig. 16 compared to a covelline / covellite standard spectrum. Settings: laser wavelength 532nm, acquisition time 10s, 5 accumulations, D2 (0.75-0.8mW), hole 500, slit 80, grating 600. Method of analysis: Raman spectroscopy, Lab of Swiss National Museum, Affoltern a. Albis ZH,6

Uniform - pitting

Type I (Robbiola)

Complementary information

Schweizer (1994) indicates that CP3 shows evidence of pseudomorphic replacement of metal grains by corrosion products that we did not observe.

Fig. 10: Stratigraphic representation of the sample taken from the pin without head in cross-section (dark field) using the MiCorr application. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation. This representation can be compared to Fig. 16, Credit HE-Arc CR.

The stratigraphies observed under binocular and cross-section are rather similar. It is not possible though to distinguish the compact underlayer under CP2 under binocular. 

The pin is made from tin bronze and has been annealed after cold working. It is covered with a regular, dense brown-yellow patina typical of lake context (Schweizer 1994). The inner, thin Sn-rich corrosion layer contains soil elements such as Si. The brown-yellow, thick intermediate and outer corrosion layers have the composition of chalcopyrite. This object was certainly abandoned rather quickly in an anaerobic, humid and S and Fe-rich environment, favouring the formation of the above mentioned compound. The limit of the original surface can be located between the chalcopyrite and the Cu depleted but Sn-rich inner corrosion layer. Thus, the corrosion is a type 1 according to Robbiola et al. 1998.

References on object and sample

References object
1. Rychner-Faraggi A-M. (1993) Hauterive – Champréveyres 9. Métal et parure au Bronze final. Archéologie neuchâteloise, 17 (Neuchâtel).
2. Hochuli, S. et al. (1988) SPM III Bronzezeit , Verlag Schweizerische Gesellschaft für Ur- und Frühgschichte Basel, 76-77, 379.

References sample
3. Empa Report 137 695/1991, P.O. Boll.
4. Rapport d'examen, Lab. Musées d'Art et d'Histoire, Geneva GE, 87-194 à 87-197.
5. Schweizer, F. (1994) Bronze objects from Lake sites: from patina to bibliography. In: Ancient and historic metals, conservation and scientific research (eds. Scott, D.A., Podany, J. and Considine B.B.), The Getty Conservation Institute, 33-50.

References on analytic methods and interpretation
6. Robbiola, L., Blengino, J-M., Fiaud, C. (1998) Morphology and mechanisms of formation of natural patinas on archaeological Cu-Sn alloys, Corrosion Science, 40, 12, 2083-2111.